This invention is concerned with high-gain projection screens in which the reflective surface comprises a two-dimensional array of small specularly reflective optical elements or lenticules, typically all of identical shape and size. By suitable selection of the optical form of the lenticule surfaces considerable control can be exerted over the distribution of the reflected light.
Such screens are potentially useful for a wide variety of projection operations in which the required viewing area subtends only a limited solid angle at the screen. By concentrating all reflective light in that viewing area, the effective brightness of the projected image can be greatly improved; and the correspondingly reduced visibility of the image outside the established viewing area may also be highly desirable on its own account. Moreover, such "containment" of the image implies the complementary feature that light reaching the screen from the sky and from other spurious sources is reflected into the viewing area only at greatly reduced intensity.
All of those characteristics are potentially valuable in motion picture drive-in theaters. Because of the large scale of the projection image that is inherently required in such theaters, it is virtually impossible to obtain fully adequate image brightness with a diffusely reflecting or other conventional screen surface; and the hours of outdoor operation are seriously limited in summer by the long hours of twilight. Also, such theaters always prefer, and are sometimes required by law, to limit visibility of the picture to the regular viewing area.
At another range of scale, the enlargement of television images by projection on a screen is presently limited by lack of image brightness. The specular reflection of a lenticular screen potentially offers a gain of several fold in efficiency of light use, and could greatly extend the utility of projection for television and other information displays.
The theoretical possibility of closely controlling the angular distribution of the reflected light from a projection screen by forming the screen surface of many reflective lenticules of suitable optical form was recognized over six decades ago by Paul L. Clark. In four patents issuing between 1914 and 1925 he analyzed some of the optical properties of such screens. Those patents are numbered U.S. Pat. Nos. 1,122,192, 1,279,262, 1,535,985 and 1,550,880, of which the first and especially the second are of primary interest. In particular, Clark showed that certain viewing areas of narrow elongated rectangular shape could be theoretically illuminated by arrays of lenticules having different vertical and horizontal curvatures corresponding to the respective dimensions of the viewing area to be served.
However, neither Clark nor others following his lead have been able to suggest any practicable way of actually producing the required optical structures with satisfactory uniformity and precision.
The practical problem of producing and mounting optically identical reflective elements or lenticules of compound curvature is made difficult by the very large number of lenticules usually required for each screen. The dimensions of each element must normally be small enough that an observer at the front of the viewing area cannot resolve individual lenticules as separate light sources. That condition is ordinarily satisfied if each lenticule measures no more than about 1/1200 of the distance from the closest viewer. For the front ramp of a drive-in theater, for example, that condition typically requires a maximum lenticule dimension of about one half inch. A typical screen surface 50 by 100 feet then needs nearly three million lenticules. Each individual lenticule must have good optical polish, must be oriented correctly within a close tolerance, and must conform accurately to a prescribed optical form suitable for the particular viewing area to be illuminated.
A further practical difficulty in producing high gain screens of the described type results from the fact that most actual viewing areas have a far larger angular dimension horizontally than vertically. Each lenticule must therefore disperse the reflected light through a larger angle horizontally than vertically in order to fill the viewing area efficiently. That is preferably done by forming the lenticules with sharper curvature in the horizontal plane than in the vertical plane, resulting in a non-spherical surface which is relatively difficult to produce with accuracy and uniformity.
Although it is possible to obtain the required greater horizontal dispersion with a spherical reflecting surface, the lenticule boundaries must be correspondingly elongated horizontally, as in U.S. Pat. No. 2,763,184 to James G. Jackson, for example. If such lenticules are made small enough to keep the horizontal dimension less than the critical distance mentioned above, the vertical dimension is less than that critical distance, making the total number and total length of the joints between lenticules greater than necessary. That makes fabrication more difficult and tends to increase the scattered light, since slight imperfections of the joints between lenticules are found to be a primary source of scattered light.
Conventional optical methods for producing lenses and mirrors are not ordinarily applicable to surfaces having different curvatures in the two dimensions. Electro-machining techniques are capable of producing highly accurate forms, but the surfaces are not specularly smooth. When such surfaces are polished the identity of shape tends to be destroyed.
One production procedure suggested in the prior art is to make a die from which a panel of elements can be molded or stamped as a unit, as suggested by Chester C. Pond in U.S. Pat. No. 2,552,455, for example. However, in machining such a die small variations tend to occur, due to wear of the machining tool and like causes. Also, it is difficult or impossible to obtain optically smooth surfaces that also conform precisely to the intended shape.